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Effi cient extraction of granitic magma from crustal sources requires the development of an extensive permeable network of melt-bearing channels during deformation. We investigate rocks that have undergone deformation and melting within the Karakoram Shear Zone of Ladakh, NW India, in which leucosome distribution is inferred to record the permeable network for magma extraction. Delicate structures preserved in these rocks record the development of this permeable magma network and its subsequent destruction to form a mobile mass of melt and solids, resulting from the interplay between folding and magma migration. During folding, magma migrated from rock pores into layer-parallel and axial-planar sheets, forming a stromatic migmatite or metatexite with two communicating sets of sheets, intersecting parallel to the fold axis. Once the network was developed, folding and stretching was eased by magma migration and slip along axial planar magma sheets. Folding and magma migration led to layer disaggregation, transposition, and the formation of a diatexite where rock coherency and banding were destroyed. A number of structures developed during this process such as cuspate fold hinges, disharmonic folds, truncated layering, shear along axial planar leucosomes, and fl ow drag and disruption of melanosomes. In this system, magma migration was an integral part of deformation and assisted the folding and stretching of metatexites, while folding gave rise to a magma sheet network, now preserved as leucosomes, as well as the pressure gradients that drove magma migration and the breakup of the metatexite. Thus, metatexite folding increased melt interconnectivity, while magma mobility increased strain rate and released differential stresses.

Granitic melt migration and pluton emplacement are commonly closely associated with transcurrent shear zones. The processes that link granites to shear zones are not yet fully understood. The dextraltranspressive Karakoram shear zone in Ladakh, NW India, exposes anatectic rocks where synkinematic melt migration and ponding at kilometer scale were controlled by competency contrasts. Metasedimentary rocks and a dominantly granodioritic calcalkaline intrusion underwent fl uid-present partial melting at upper-amphibolite facies to produce leucogranite sheets and irregular intrusive masses dated at 21-14 Ma. Leucogranitic magmas ponded in the lowpressure strain shadow of the competent grano dioritic calc-alkaline pluton, giving rise to (a) migmatitic rocks that are pervaded by irregular leucogranite intrusions at a scale of meters or tens of meters, and (b) the growth of the Tangtse pluton, a kilometer-scale sheeted complex. Thus, magmas accumulated during shearing and anatexis in a low-pressure strain shadow within the Karakoram shear zone. This magma provided a readily available magma source that could have been tapped to feed larger plutons at shallower levels by modifi cations in the pressure distribution accompanying changes in shear zone geometry and kinematics. We conclude that shear zones tapping anatectic regions act as magma pumps, creating and destroying magma traps at depth as they evolve, and leading to incremental magma addition to upper-crustal plutons.

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